When I tank stuff, I will use the Prismatic/Mastery combo if resistable Magic damage is somewhere above 20% or so of total tank damage taken and if it is what is putting me most at risk of death. I figure, resisted damage taken vs ~ 4% (or less) health from stam flask. If I am using the Mastery elixir, then I generally also adjust my trinkets/food buffs so as not to be too far over CTC.

I believe Theck once worked out some kind of breakeven back in T11 a little more formally, but at some point the prismatic elixir will reduce overall damage taken by more then a stam flask will increase health pool, and that is when I tent to like the elixirs. Yor'sahj in particular is a good example of this. Zon'ozz has heavy magic damage, but he can hit like a truck too as he builds stacks, so a good physical defense is still quite important.